1897.] NEW YORK ACADEMY OF SCIENCES. 315 



Now the result of the change in the position of the antero- 

 posterior axis and of the axis of motion has made a mechanical 

 demand for opponents to the anterior and posterior adductors, 

 and the important observation is that such muscles have arisen 

 by adaptation of pallial muscles at just the points where one 

 would expect that thej would arise. This fact gives ver3' strong 

 support to the opinion that the adductors of Mya and the like 

 have arisen in just this way. 



Conversely, in Teredo, in which the axis of motion is con- 

 stant in a dorsi-ventral position, and in which the anterior ad- 

 ductor is wholly anterior to the axis of motion, and, therefore, 

 to the umbonal point of contact, there has been no demand for 

 a ventral adductor and none for an accessory anterior adductor; 

 andneither of these is found. I have said that the posterior adduc- 

 tor, by its extent, partly compensates for the absence of the ven- 

 tral adductor, but this is true only in that it gives to the valves 

 a mutual stability which the ventral adductor gives in the other 

 forms. 



A remarkable character which is coordinate with the presence 

 of this musculature is the absence of the hinge ligament. This 

 structure is to be found only in lamellibranchs in which the 

 axis of motion is constant in position, and its function, as is 

 well known, is to cause the gaping of the valves, or in other 

 words, to oppose the adductor muscles. It is evident that the 

 presence of the hinge ligament in the piddocks would render im- 

 possible the sea-saw movement of the valves about an axis of 

 motion having a dorsi-ventral position. Not only this, for on 

 account of the passive character of the same it would be useless 

 in boring ; for the amount of latent force in the compressed liga- 

 ment is by no means as great as that which a muscle of equiva- 

 lent size is capable of exerting. 



Now the absence of the hinge ligament is more than compen- 

 sated for by the presence of the anterior adductor muscle, which 

 is attached to the outside of the shell surrounding the umbones. 

 The muscle is preceded in its passage from the interior to the 

 exterior of the shell by an outgrowth of the mantle which over- 

 lays the rough exterior of the shell with a mother-of-pearl layer, 

 and secretes at its posterior extremity the triangular accessory 

 piece. It would seem that this change of position is effected by 

 the addition of muscle fibers to that side of the muscle which is 

 of maximum use in the animal's endeavor to produce such a 

 motion of the valves as has been described, and by their atrophy 

 on the opposite side. In this wa}' the complete homology is 

 preserved. It may be, however, that an entirely new mass of 

 muscular tissue arises in the mantle distinct from the original 



